One of the more important constraints to the continued growth of air traffic is the related concern regarding aircraft noise. This concern has resulted in increasingly stringent noise restrictions for airports. During aircraft take-off, the dominant aircraft noise source is generally the propulsion noise from the engines of the aircraft. During aircraft approach and landing, airframe noise becomes a prominent component on par with the engine noise. This airframe noise is caused by the interaction of the unsteady and typically turbulent airflow with the aircraft structures. The sound radiated from the side edge of a partial-span flap is one of the major contributors to airframe noise during aircraft approach and landing.
Acoustic liners have been used to absorb propulsion noise within aircraft engine nacelles. Acoustic liners with variable-depth chambers to achieve broadband noise attenuation have been known for some time.